Much electronic data processing equipment, e.g. telephones, computers and facsimile machines, makes use of 4-wire digital signals (often referred to in North America as digital data services, or DDSs). Transmission of such signals requires four wires which have no abrupt changes in impedance. For example, if existing telephone lines are to be used for 4-wire digital services, they must undergo testing and modification (often referred to as Class C3 data conditioning). This is a serious disadvantage, even if four telephone lines are available, which is not always the case. In addition, repeaters are often needed. For example, one typical 4-wire signal is a bipolar signal at a transmission rate of up to 56 kbps. Such signals typically need a repeater if the line span exceeds 3000 to 3,650 meters (10,000 to 12,000 feet). A further difficulty is that it is difficult to combine conventional analog telephone service (POTS) with 4-wire digital service.